examples: Fix boid_flockers viz (projectmesa#2492)

Boids didn't have neighbours anymore, so the visualisation crashes. This commit fixes that.

mesa/examples/basic/boid_flockers/agents.py

@@ -54,13 +54,14 @@ def __init__(
         self.cohere_factor = cohere
         self.separate_factor = separate
         self.match_factor = match
+        self.neighbors = []
 
     def step(self):
         """Get the Boid's neighbors, compute the new vector, and move accordingly."""
-        neighbors = self.model.space.get_neighbors(self.pos, self.vision, False)
+        self.neighbors = self.model.space.get_neighbors(self.pos, self.vision, False)
 
         # If no neighbors, maintain current direction
-        if not neighbors:
+        if not self.neighbors:
             new_pos = self.pos + self.direction * self.speed
             self.model.space.move_agent(self, new_pos)
             return
@@ -71,7 +72,7 @@ def step(self):
         separation_vector = np.zeros(2)  # Separation vector
 
         # Calculate the contribution of each neighbor to the three behaviors
-        for neighbor in neighbors:
+        for neighbor in self.neighbors:
             heading = self.model.space.get_heading(self.pos, neighbor.pos)
             distance = self.model.space.get_distance(self.pos, neighbor.pos)
 
@@ -86,7 +87,7 @@ def step(self):
             match_vector += neighbor.direction
 
         # Weight each behavior by its factor and normalize by number of neighbors
-        n = len(neighbors)
+        n = len(self.neighbors)
         cohere = cohere * self.cohere_factor
         separation_vector = separation_vector * self.separate_factor
         match_vector = match_vector * self.match_factor

mesa/examples/basic/boid_flockers/app.py

@@ -3,10 +3,7 @@
 
 
 def boid_draw(agent):
-    if not agent.neighbors:  # Only for the first Frame
-        neighbors = len(agent.model.space.get_neighbors(agent.pos, agent.vision, False))
-    else:
-        neighbors = len(agent.neighbors)
+    neighbors = len(agent.neighbors)
 
     if neighbors <= 1:
         return {"color": "red", "size": 20}